Because of their excellent throwing power and low environmental impact, cationic electrodeposition coating compositions are widely used for various applications such as undercoating compositions for automobiles etc. Cationic electrodeposition coating compositions that contain lead compounds and/or chromium compounds, e.g., lead chromate, basic lead silicate, and strontium chromate, have been suggested.
However, because of problems with environmental pollution, the use of toxic compounds such as lead compounds and chromium compounds is restricted; and a cationic electrodeposition coating composition that uses a non-toxic or low-toxic rust-preventive pigment, and provides excellent anti-corrosion properties without containing harmful compounds has been developed and is now in use.
For example, Patent Literature 1 discloses a cationic electrodeposition coating composition that comprises (A) a cationic amine modified epoxy resin, (B) a blocked polyisocyanate, and (C) a phosphorous acid divalent or trivalent metal salt, the purpose thereof being to improve anti-corrosion properties without containing lead. Patent Literature 2 discloses a zirconium compound-containing cationic electrodeposition coating composition that exhibits excellent anti-corrosion properties without containing lead.
Patent Literatures 1 and 2 disclose that a film formed on a surface-treated steel sheet has excellent anti-corrosion properties even without containing toxic substances such as lead compounds and chromium compounds. However, an electrodeposition coating film formed on a surface-untreated steel sheet has insufficient anti-corrosion properties.
Patent Literature 3 discloses a cationic electrodeposition coating composition comprising a cationic amine modified epoxy resin, a blocked polyisocyanate, and a zirconium salt, the purpose thereof being to improve rust preventive properties to the greatest extent possible. Patent Literature 3 teaches that the composition can provide, even without containing harmful compounds such as lead compounds, chromium compounds, etc., an electrodeposition coating film having excellent anti-corrosion properties on a surface-untreated steel sheet.
However, under severely corrosive conditions, the aforementioned electrodeposition coating film formed on the surface-untreated steel sheet exhibits insufficient anti-corrosion properties; in particular, an electrodeposition coating film having a thickness of 15 μm or less (when dried) that is formed on an untreated steel sheet exhibits insufficient anti-corrosion properties. Accordingly, further improvement has been desired.